Brothers Grimm which spawned numerous derivatives by Disney and others. But, as I have learned over many years of collecting books about German regional folklore, these barely scratch the surface - German folklore is much stranger and even outright bizarre than the tame, made-for-children animated movies hint, and much of it is ready-made for gaming.
Unfortunately, while there are numerous books on regional folklore, myths and legends, few of these books are available in English. Which is where I come in - I plan to read through these books, and as I do so take notes on particularly interesting gaming material. But where to begin?
At home - or to be more precise, a region near where I grew up. This is the Fränkische Schweiz, which Wikipedia translates literally as "Franconian Switzerland" - a phrase I won't use henceforth, as it sounds rather daft in English (incidentally, the current name for the region was only established early in the 19th century, when people all over the German-speaking lands started to call local hilly areas "Schweiz", or "Switzerland", for marketing reasons in order to attract tourists).
The Fränkische Schweiz is a hill/low mountain region in Franconia (the northern half of Bavaria), roughly occupying the space between the cities of Bayreuth, Bamberg, and my hometown of Erlangen. It has an extremely high density of castles - about 200 (most of which are now in ruins or completely gone) were once scattered through this region, many of which once housed infamous families of Raubritter ("robber knights"), preying on the surrounding trade routes, especially those connecting to the nearby trade center of Nuremberg. Due to its chaotic terrain, the region was never really unified, with the surrounding cities vying for influence. To top it off, the region is dotted with innumerable caves.
In other words, the Fränkische Schweiz would be a prime ground for adventures even if you were to ignore the supernatural elements - which we won't, of course. My guide to this aspect of the region will be "Sagen Legenden und Geschichten aus der Fränkischen Schweiz" by Heinz Büttner (which you can order here, and tell them I sent you!).